A fraction bar is a horizontal line that separates the numerator and denominator in a fraction. It visually represents the division of the numerator (the top number) by the denominator (the bottom number). For example, in the fraction 3/4, the fraction bar indicates that 3 is divided by 4. Fraction bars help in understanding and comparing fractions. They make it easier to see how different fractions relate to each other, such as when adding or subtracting them. Using a fraction bar can also aid in simplifying fractions, making it a useful tool in mathematics.